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Basic and Applied Ecology 12 (2011) 1–9
Plant invasions in temperate forests: Resistance or ephemeral
phenomenon?
Franz Essla,∗ , Norbert Milasowszkyb , Thomas Dirnböcka
a
b
Environment Agency Austria, Spittelauer Lände 5, A-1090 Vienna, Austria
Institut für angewandte Biologie und Umweltbildung (IFABU), Argentinierstraße 54/21, A-1040 Vienna, Austria
Received 23 March 2010; accepted 13 October 2010
Abstract
Invasion patterns in Europe are uneven across habitats. In particular, temperate zonal forests are relatively little affected by
invasions. However, long generation periods of forest species and the rarity of disturbance events may lead to long time lags,
and so, invasion resistance of temperate forests may have been overestimated.
Based on the inventory of the alien vascular flora of 15 study sites in East Austrian lowland forest patches adjacent to villages,
we analysed diversity and temporal trends of their alien flora and the influence of within-habitat diversity, occurrence of forest
areas dominated by alien tree species and adjacent land use diversity on the level of invasion.
In total, we recorded 119 alien (neophyte) species. Species introduced by horticulture are of overriding importance (86% of
all aliens). Using the year of first record in Austria as a proxy for local residence time, numbers of aliens have increased linearly
by five species per decade since 1800. A GLM including the study site size and ratio of forest areas dominated by alien vs.
native tree species explains 64% of the variance of the number of alien species. Size was more important, but the ratio of forest
areas dominated by alien trees (Ailanthus altissima, Robinia pseudacacia) significantly increased alien species number.
Our results show, that invasion of alien trees with strong impacts on ecosystem properties facilitates invasion in Central
European forests, leading to “invasional meltdown”. Further, high levels of propagule pressure, created by adjacent settlements
and gardens, may foster invasions. Currently observed low levels of invasion in Central European forests situated in greater
distance to settlements may turn out to be an ephemeral phenomenon. Spread of alien forest plants on the landscape level may
ultimately, although possibly only over long time periods, lead to increased levels of invasion.
Zusammenfassung
Neophyten besiedeln unterschiedliche Lebensräume Europas in unterschiedlichem Ausmaß. Im Besonderen gelten temperate zonale Wälder als nur wenig von biologischen Invasionen betroffen. Allerdings können die lange Generationszeit von
Waldarten und die Seltenheit von Störungserereignissen zu besonders langen Verzögerungen von Invasionen führen, sodass die
Invasionsresistenz temperater Wälder möglicherweise überschätzt wird.
Auf Basis eines Inventars der Neophytenflora von 15 an Siedlungen angrenzenden Waldflächen im östlichen Tiefland
Österreichs untersuchten wir Diversität und zeitliche Trends der Neophytenflora sowie die Bedeutung der Habitatvielfalt,
des Vorkommens von durch nichtheimische Baumarten dominierten Waldflächen und angrenzender Landnutzung.
∗ Corresponding
author. Tel.: +43 1 31303 3323; fax: +43 1 31304 3700.
E-mail address: franz.essl@umweltbundesamt.at (F. Essl).
1439-1791/$ – see front matter © 2010 Gesellschaft für Ökologie. Published by Elsevier GmbH. All rights reserved.
doi:10.1016/j.baae.2010.10.003
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F. Essl et al. / Basic and Applied Ecology 12 (2011) 1–9
Insgesamt fanden wir 119 Neophytentaxa. Aus Gartenkultur verwilderte Arten dominierten (86% aller Neophyten). Auf Basis
des Erstnachweises der Arten in Österreich als Maßzahl für lokale Anwesenheitszeit wuchs die Neophytenflora mit einer linearen
Zunahme von 5 Arten pro Dekade seit 1800 an. In den letzten Jahrzehnten haben Sträucher, aus Asien stammende Arten, in
Gartenkultur entstandene Sippen (Anökophyten) und als Gartenpflanzen eingeführte Arten überproportional zugenommen.
Ein GLM mit Flächengröße und dem Anteil von durch nichtheimische Baumarten dominierten Waldflächen erklärt 64%
der Varianz der Neophytenzahl. Flächengröße war wichtiger, aber der Anteil von durch nichtheimische Baumarten (Ailanthus
altissima, Robinia pseudacacia) dominierten Waldflächen erhöhte ebenfalls die Neophytenzahl signifikant.
Unsere Ergebnisse zeigen, dass das Eindringen nichtheimischer Baumarten mit starken ökosystemaren Auswirkungen die
Ausbreitung anderer Neophyten in mitteleuropäischen Wäldern begünstigt und zu einem “invasional meltdown” führt. Angrenzende Siedlungen und Gärten scheinen wegen des hohen Samendrucks Invasionen zu begünstigen. Die derzeit beobachtete
geringe Ausbreitung von Neophyten in mitteleuropäischen Wäldern in größerem Abstand zu Siedlungen mag sich daher zukünftig als ein temporäres Phänomen herausstellen. Die Ausbreitung von Neophyten auf der Landschaftsebene mag letztlich,
wenngleich vermutlich über längere Zeiträume, zu einem höheren Invasionsniveau führen.
© 2010 Gesellschaft für Ökologie. Published by Elsevier GmbH. All rights reserved.
Keywords: Central Europe; Habitat diversity; Horticulture; Invasional meltdown; Life form; Pathway; Propagule pressure; Residence time;
Temporal change; Vascular plants
Introduction
The rate of biological invasions is rapidly increasing
worldwide (Lambdon et al. 2008; Hulme, Pyšek, Nentwig,
& Vilá 2009). Invasive species (transformer species sensu
Richardson et al. 2000) are causing large detrimental effects
on both biodiversity and human well-being (Vilà et al. 2009).
Recent research has demonstrated that, besides climate
matching, human activities are an important determinant
of the biogeography of invasions (Taylor & Irwin 2004;
Hulme 2009). In particular, human population size and
various indicators of socio-economic behaviour have been
shown to be linked to patterns of invasion (McKinney
2001; Pyšek, Kucera, & Jarošik 2002; Taylor & Irwin 2004;
Hulme 2009), because they are surrogates of propagule pressure and changed disturbance regimes of natural systems
(McKinney 2001; Lockwood, Cassey, & Blackburn 2005;
Colautti, Grigorovich, & MacIsaac 2006; Krivánek, Pyšek,
& Jarošik 2006).
Biological invasions are often characterized by time lags
between first introduction of a species in a new territory and
the start of invasive spread (Kowarik 1995; Richardson et al.
2000). The duration of such time lag phenomena largely
depends on species traits (e.g. life cycle), introduction history (e.g. amount of propagules or individuals introduced)
and matching of the alien species autecology with the habitat
conditions in the new territory (Pyšek & Richardson 2007;
Bucharova & van Kleunen 2009; Pyšek, Krivánek, & Jarošik
2009) or the time necessary for phenological or genetic adaptations to new abiotic and biotic environments. For long-lived
species like shrubs or trees such time lags may be substantial,
reaching decades or centuries (Kowarik 1992, 1995; Pyšek
et al. 2009, but see Daehler 2009).
Invasion patterns in Europe are inconsistent across habitats (Chytrý et al. 2008). So far, temperate zonal forests are
relatively little affected by invasions (Chytrý, Pyšek, Tichy,
Knollova, & Danihelka 2005; Chytrý et al. 2008; Walter, Essl,
Englisch, & Kiehn 2005). However, taking into account that
forests are dominated by woody species with long generation
periods and invasion into forests predominantly occurs when
disturbances create gaps or clearings (Chytrý et al. 2005,
2008) leading to temporarily increased resource availability,
which can foster plant invasions (Davis, Grime, & Thompson
2000), invasions in temperate forests may exhibit particularly long time lags. Hence, invasion resistance of temperate
forests might have been overestimated.
Based on the inventory of the alien vascular flora of 15
study sites in East Austrian lowland forest patches, we ask
the following questions: (1) What is the diversity, structure
and composition of the alien flora? (2) What are the temporal trends (time elapsed since the invasion of the species in
Austria) in alien species composition and is there a change
over time in the importance of pathways, regions of origin
and life forms? (3) What are the effects of within-habitat
diversity, occurrence of forest areas dominated by alien tree
species and adjacent land use type diversity on the level of
invasion?
Data
Study area
The study area is situated in the Marchfeld region in
the lowlands of Eastern Austria, 30 km east of Vienna
(48◦ 17′ –48◦ 20′ N, 16◦ 35′ –16◦ 43′ E, Fig. 1). The flat area is
part of the glacial Danube valley (165 m a.s.l.). The bedrock
consists of gravel deposited by the Danube during the Ice
Ages, which is covered by sandy alluvial soils. The climate
is temperate pannonic, with cool winters and warm summers,
average annual temperature being 10 ◦ C and annual precipitation 550–600 mm (1961–1990, Wiesbauer & Mazucco 1997).
The Marchfeld is dominated by agriculture, so only few significant forests remain. However, in the central part of the
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F. Essl et al. / Basic and Applied Ecology 12 (2011) 1–9
Fig. 1. Location of the study area in East Austria and of the 15
forest patches within the central Marchfeld region. ©BEV 2010
(T2010/70002).
Marchfeld, several extensive forests of approximately 10 km2
exist in the environs of the villages of Deutsch Wagram,
Straßhof and Gänserndorf. These forests have been replanted
on formerly extensively grazed dry grassland in the 18th and
19th century (Wiesbauer & Mazucco 1997). Due to their
poor growth, they are currently not or only extensively used
silviculturally.
Species data
In the years 2008 and 2009, the alien vascular flora of 15
forest patches of different sizes (ranging from 2.4 ha to 47 ha)
of the central Marchfeld was recorded (Fig. 1). The study sites
are separated from each other by settlements and agricultural
fields, and the minimum distance between individual study
sites is 150 m. All sites are dominated by old-growth forest
(stand age > 60 years), only small fractions of the sites are
occupied by medium-aged forest (minimum stand age: 30
years), whereas young-growth areas are of negligible size.
Due to their uniform abiotic characteristics (flat topography,
identical soil types and climate) and history, these forest sites
offer a unique possibility for a comparative assessment of
invasions.
For each forest patch, a set of variables was collected (see
Table S1). Based on the digital AustrianMap (www.amap.at),
we calculated the size of each site. Based on field data and
aerial photos, we mapped and calculated the percentage of
three different forest types to account for habitat diversity
and forestry intensity within the sites: native broadleaved
forests (for.nb) dominated by native oaks (Quercus cerris, Q.
petraea, Q. robur), native conifer plantations (for.ncp) dominated by pine species native to eastern Austria (Pinus nigra,
P. sylvestris), and forests dominated by alien broadleaved
trees (Ailanthus altissima, Robinia pseudacacia; for.ab). To
account for the influence of adjacent habitat diversity, the
presence/absence of the following four habitats was recorded:
large (two lane) roads (l.s.), railway (r.w.), crop fields (fi.),
built-up areas (b.u.).
We collated a complete list of alien vascular plants, the frequency was assessed in five frequency classes (see Table S2).
3
For analyses, we used presence/absence data only. We considered only alien species recorded after 1500 in Austria
(neophytes), while old aliens (archaeophytes) were excluded
(Richardson et al. 2000). To sample a species list as complete
as possible, each site was visited at least four times in different
months of the growing season to cover different phenological periods. To avoid size-related sampling biases, surveys
in larger sites lasted longer, with total sampling effort being
approximately 1 h per 4 ha. Data collection was done by systematically surveying all parts of the patches, while excluding
forest edges. Taxonomically critical species were collected
and verified or determined by specialists (see “Acknowledgements”). A few species could only be identified to genus level,
but they were treated as species in the analyses.
For each species, we collated a set of additional variables: first record in Austria (as a proxy for the local
residence time), region of origin (continents), and dominant pathways (intentional introduction by horticulture
[including ornamental trees] or forestry; unintentional introduction) based on the information provided in AliensAustria
(2009). All species were assigned to life form categories:
macro-phanerophytes (trees), nano-phanerophytes (shrubs),
long-lived herbs (hemicryptophytes, geophytes) and short
lived herbs (annuals and biennals). Based on the database
AliensAustria (2009), each species was classified according to its habitat affiliation as forest species or as non-forest
species. The latter mainly consist of ruderal and grassland
species which occur in small gaps, along forest edges or tracks
inside forests.
Taxonomy and nomenclature follow Fischer, Adler, and
Oswald (2008), for alien species not covered there, we followed the database AliensAustria (2009).
Statistical analyses
When testing for the influence of within-habitat and adjacent land type diversity, we used two approaches in order to
account for the different sizes of the forest patches. First, log
area was used as a covariate in the analysis. The independent
variables are thus related to the residual variation of the diversity by size model. Second, all independent variables were
standardized for the area effect by dividing through size.
Two variables were used, which are assumed to determine
the variation in species richness: Forest habitat diversity of
the sites was expressed as the number of forest habitat types
divided by area of the respective site (div.hab). To analyse
the effects of adjacent habitat diversity, the presence and
absence of adjacent habitats were summed and then divided
by the size of the respective site (div.a). Since the number
of alien species was the response, we used generalised linear models (GLM) from the Poisson family with a log-link
function. Significance was tested by dropping the respective
independent variable from the model with the size as a covariate using the likelihood ratio statistic and assuming a χ2 -test
distribution under the null hypothesis. In addition, quasipois-
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F. Essl et al. / Basic and Applied Ecology 12 (2011) 1–9
Table 1. Numbers of alien species of 15 forest patches as explained
by the ratio of forests areas dominated by alien vs. native tree
species within each forest patch (for.r) using generalized linear
models (GLM) with Poisson distribution and a log-link function
(null deviance: 46.9, residual deviance: 17.2). The log of the size
of the forest patch is used as a covariate to control for area-related
species richness. Significance is determined by dropping the respective variable from the full model using the likelihood ratio statistic
(LR) assuming a χ2 distribution. df, dev: the change in the degrees of
freedom and the deviance respectively when dropping the respective
variable from the full model; coeff: coefficient.
log (size)
for.r
df
dev
coeff
AIC
LR
p-Value
1
1
40.9
20.4
+0.25
−0.44
127.9
107.3
23.8
3.2
<0.001
0.073
son models were used accounting for overdispersion but as
the results were the same, we here show the simpler Poisson models. Due to the limited number of investigated forest
patches separate models were used for: (1) the ratio (for.r)
between area dominated by alien (for.ab) and area dominated
by native (for.ncb + for.nb) tree species within each forest
patch. This ratio ranges between 0 and 0.7, because area of
forests dominated by native tree species is higher than forest
area dominated by alien trees in each site; (2) the diversity of
habitats within (div.hab) and adjacent to (div.a) the respective
forest patch. As a result of the limited sample size we accept
the alternative hypotheses at an ␣-level of 0.1.
We tested the effect of residence time of the alien species
in Austria on their abundance on the local scale (number of
invaded forest patches) using a Kruskal–Wallis–H test. To
account for differences in invasion success of alien species
of different habitat affiliations, we did this analysis separately for forest and non-forest species. We analysed temporal
changes in the composition of the alien flora by testing for
differences of the residence time in Austria for different pathways, regions of origin and life forms by means of KW-tests.
Statistical analyses were carried out in SPSS Version
11.5.1 (Anonymous 2002) and R (R Development Core Team
2009).
Results
Diversity, structure and composition of the alien
flora
In total, we recorded 119 alien (neophyte) species in the
15 study sites (see Table S2). Six species were recorded in
all sites, whereas 51 species were rare (recorded at most in
two sites). Alien species numbers per site ranged from 25
to 69 species. Size of the forest patch has a large impact
on alien species number (Table 1). The most important life
form present are long-lived herbs (36% of all alien species),
followed by shrubs (27%), short-lived herbs and trees (19%
each). Tree species, however, colonized on average the highest number of sites (8.5 sites), followed by shrubs (5.7 sites),
short-lived herbs (4.0 sites) and long-lived herbs (3.9 sites).
North America, Europe and Asia are of nearly equal importance as regions of origin (29–32 species), and 14 species have
originated in cultivation (anecophytes). No species native to
other continents were found. Species introduced intentionally
are by far the largest group (88% of all aliens). The pathway
of overriding importance is horticulture (86% of all aliens),
whereas only 2% of the species have been introduced for
silviculture.
Temporal change in composition of life forms,
pathways and regions of origin
Using the year of first record in Austria as a proxy for local
residence time, invasion rate has been low until 1800. Since
then, the number of aliens has accumulated much faster with
a linear increase of 5 species per decade (Fig. 2A). Species
introduced after 1900 into Austria make up 50% of all alien
species recorded.
Alien species of different life forms significantly differ
in their residence time (p < 0.001, KW-test). Shrubs are the
most recent invaders compared to aliens of other life forms
(Figs. 2A and 3A). European and North American species
have been introduced earlier into Austria, whereas species
native to Asia or originated in cultivation are more recent
introductions (Fig. 2B) (p = 0.016, KW-test).
Importance of pathways has changed over time
(Figs. 2C and 3B). Species introduced unintentionally are
predominantly old introductions (86% introduced before
1900 into Austria), whereas only 46% of the species introduced for horticultural purposes have been recorded in
Austria before 1900. Temporal differences between pathways
are highly significant (p = 0.005, KW-test).
Residence time of the species in Austria had a significant positive influence on the number of forest patches
invaded (Fig. 4); this is true for forest and non-forest species
(p < 0.001, KW-test), although for the latter the numbers of
invaded forest patches are significantly lower at all time
periods. Forest species were recorded throughout the forest
sites, whereas non-forest species tended to invade gaps, forest
tracks and young-growth areas.
The effect of within and adjacent habitat
diversity
The GLM including size and ratio of alien vs. native forests
explains 64% of the variance of the number of alien species.
Size was clearly more important, but the ratio significantly
contributes to the explained variance (p < 0.1; Table 1). A
negative coefficient of −0.44 means that with every 10%
more of alien forest area per site, approximately 1.6 alien
plant species are added to the forest vegetation.
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F. Essl et al. / Basic and Applied Ecology 12 (2011) 1–9
5
Fig. 2. Boxplots of the year of first record in Austria as a proxy for local residence time of alien species classified by (A) life form (ll = long-lived
herbs, sh = shrubs, sl = short-lived herbs, tr = trees), (B) region of origin (AS = Asia, cult = originated in cultivation, EU = Europe, NA = North
America), and (C) pathway (for = alien species introduced for forestry, hor = species introduced for horticultural reasons, unint = species
introduced unintentionally).
Both, within-site habitat diversity and adjacent land use
diversity did not significantly explain the number of alien
species in the second GLM. In addition, we tested the
diversity variables in separate models and as additional independent variables in the above models, but the p-values
always remained >0.1.
Discussion
Characteristics of the alien flora
The number of alien species recorded in the 15 forest
patches is exceptionally high, making up 11% of the total
Fig. 3. Cumulative species numbers (using year of first record in Austria as a proxy for local residence time) for 15 forest patches (A).
Cumulative species numbers for alien species of different life forms (A) and for alien species introduced by different pathways (B).
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F. Essl et al. / Basic and Applied Ecology 12 (2011) 1–9
ions (e.g. recent wide spread planting of ornamental shrubs)
have contributed to this pattern, as both processes enhance
invasion velocity at the landscape scale (Kowarik 2003b).
Temporal changes in alien species composition
Fig. 4. Number of forest patches (total n = 15) invaded by alien
forest (grey bars) and non-forest (white bars) species classified by
time of arrival in Austria (year of first record, 5 periods).
alien flora of Austria (1110 alien vascular plants, Rabitsch &
Essl, 2006) on a total area of 307.2 ha. In addition, this species
number was recorded in forests only. Besides the proximity
of extensive settlements and gardens leading to high propagule pressure, another reason for this high level of invasion
may be attributed to the mild climate of the study region, as
levels of invasion of aliens in Central Europe are higher in
the lowlands (Pyšek et al. 2002; Chytrý et al. 2008).
Although the Austrian alien flora is exceptionally well documented (Essl & Rabitsch 2002; AliensAustria 2009), several
of the alien species recorded have not or only very rarely
been found before in Austria (e.g. Amelanchier lamarckii,
Anemone blanda, Fritillaria imperialis, Kolkwitzia amabilis,
Ribes multiflorum) and testify the dynamics of alien species
spread in Europe (Lambdon et al. 2008; DAISIE 2009).
Furthermore, several species recorded only recently for the
first time in Austria are wide-spread in our forest patches
(exclusively shrubs as Cotoneaster divaricatus, C. dielsianus,
Kolkwitzia amabilis, Ribes multiflorum, see Table S2). The
incipient invasion of evergreen broadleaved shrubs and trees
in the warmest parts of Central Europe due to global warming (Walther 2002; Walther et al. 2009) was confirmed, as
several species, most of them being rare and evidently only
recently escaped, were recorded (Euonymus fortunei, Viburnum rhytidophyllum). Escaped fruit and nut trees of adjacent
gardens (e.g. Juglans regia, Malus domestica, Morus alba,
Prunus spp., Pyrus communis) are conspicuously common.
Interestingly, species of those life forms with the longest
generation periods (trees, shrubs) occurred on average on
more sites than species with shorter generation periods,
although they are on average no longer (trees) or even for
a shorter time recorded as alien in Austria. We argue that
zoochory (many shrubs and trees belong to species dispersed
by birds or mammals, e.g. Cotoneaster spp., Juglans regia,
Prunus spp., Ribes spp.), and changes in horticultural fash-
We used year of first record in Austria as a proxy for local
residence time. This approach has been used widely in assessments of the influence of residence time on invasion success
of alien species (e.g. Castro, Figueroa, Munoz-Schick, &
Jaksic 2005; Pyšek & Jarošik 2005; Essl & Dirnböck 2008)
and we consider it a robust measure of temporal patterns of
invasion at the local scale, although it also bears its limitations (e.g. local invasion may in fact have started significantly
later than on the national level). Using this proxy we infer
that the temporal changes in composition of the alien flora
are substantial with regard to regions of origin, life forms
and pathways. In the last decades, shrubs, species native to
Asia, originated in cultivation, or introduced by horticulture
have increased disproportionally in the alien flora of the study
sites.
The overriding importance of ornamentals in the alien flora
of the studied forest patches (86% compared to 57% of the
national alien vascular plant flora, Essl & Rabitsch 2002) may
be attributed to the combined effects of several factors. First,
in Europe (Hulme et al. 2008; Lambdon et al. 2008) and Austria as well, horticulture is by far the most important pathway
for alien vascular plants. Second, they mostly are and were
introduced repeatedly into new regions, and horticultural
usage of these species in different environments increases
propagule pressure and the likelihood that these species will
escape (Kowarik 2003b; Dehnen-Schmutz, Touza, Perrings,
& Williamson 2007a, 2007b; Bucharova & van Kleunen
2009). All of our study sites are adjacent to settlements with
gardens constructed in the 1950s to 1980s, where planting of
ornamentals and the deposition of garden wastes in adjacent
forests increase local levels of propagule pressure. Third, the
numbers of cultivated ornamentals increased strongly in the
last decades (Kowarik 2003a; Dehnen-Schmutz et al. 2007b).
Predominance of early introductions in European and
North American species compared to those originating
from Asia or from horticulture indicates the importance of
changes in the structure of international trade (Hulme et al.
2008; Hulme 2009). General and horticultural trade between
Europe and temperate East Asia – the dominant region of
origin for Asian origin – has belatedly gained momentum
(Mitchell 2003; Hulme 2009). Species which have originated
in cultivation mostly as results of intensive breeding efforts,
have in several cases been created only during the last decades
(e.g. Fallopia × bohemica, Galeobdolon argentatum). On the
other hand, species native to adjacent regions of Europe or to
the former European colonies in North America were introduced earlier, giving them a headstart.
As expected and in concordance with recent studies on
plant invasions in Europe (Pyšek et al. 2009; Williamson et al.
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F. Essl et al. / Basic and Applied Ecology 12 (2011) 1–9
2009), residence time of the species in Austria had a significant effect on the number of invaded forest patches. Numbers
of invaded forest patches increased stronger with residence
time for forest species, and non-forest species invaded fewer
forest patches, which reflects the fact, that the latter find only
very localized appropriate habitats in small forest gaps or
along tracks.
The late date of first records of shrubs in Austria compared
to other life forms was unexpected and may partly result from
changing fashions in horticulture (e.g. recently increased
plantation of ornamental shrub species of Cotoneaster spp.
and Ribes spp.).
Role of within-habitat and adjacent land use
diversity
As expected by biogeographic theory and empirical evidence (e.g. McKinney 2001; Pyšek et al. 2002; Essl &
Dirnböck 2008), patch size explained a large part of the variance. Second to this variable, only the proportion of alien
forests contributed to the number of alien species. That is,
the presence and extent of forests dominated by Ailanthus
altissima and Robinia pseudacacia contributed to higher
numbers of alien species. Both species form dense stands,
and particularly the latter one, which is also more widespread
in the study sites, has severe impacts on ecosystem properties, especially by altering the nutrient cycle (Kowarik 2003b;
Kowarik & Säumel 2007; Kleinbauer, Dullinger, Peterseil, &
Essl 2009). A recent study for Austria has shown, that Robinia
pseudacacia is currently invading a wide range of habitat
types (including oak forests, the dominant habitat type in our
study region) and that climate change will increase invasion
risks substantially across all habitat types (Kleinbauer et al.
2009). Hence, both climate change and secondary releases
by planting for forestry or landscaping (Kowarik 2003b) will
contribute to future spread of Robinia pseudacacia in the
forest patches.
Our results show, that invasion of alien trees with strong
impacts on ecosystem properties, facilitates invasion in Central European forests – a feature, which has been aptly termed
“invasional meltdown” (Simberloff & Von Holle 1999). Surprisingly, within and adjacent land use did not influence
the numbers of alien species. We argue that this is due to
the fact that all sites were adjacent to gardens and built-up
areas which led to high levels of local propagule pressure
and which masked possible influences of other land use
types.
7
invasions (Lambdon et al. 2008; Chytrý et al. 2008), and
there are only few substantial and wide spread invasions
(Chytrý et al. 2005; Walter et al. 2005). The cause of this
paucity of invasions is primarily seen as a lack of available resources and strong interspecific competition (Davis
et al. 2000), which limit recruitment and the establishment
of alien species (Chytrý et al. 2008). However, as forests
are dominated by long-lived woody species, time lag phenomena stemming from long intervals between disturbances
(e.g. gaps created by windfalls or tree harvesting), and dispersal limitation (e.g. aggravated by isolation of forest patches)
may be of particular relevance (Svenning & Skov 2004,
2007), as they may delay alien species spread and hence
mask invasibility of forests. Additional data from transects
laid through the study sites (Essl unpubl.) show that there
are no significant correlations between stand age and numbers of alien species recorded. So, we conclude, that in
forest succession numbers of alien species in the investigated
forests does not decline significantly as forest succession
proceeds (e.g. as a consequence of shading and lack of disturbance).
Conclusions
Our case study shows, that temperate forests are not
resistant to invasions. High levels of propagule pressure,
created by adjacent settlements and gardens, may lead to
high and increasing levels of invasion, which ultimately may
profoundly change the structure and composition of the vegetation. Further, invasion of alien species with strong impact
on ecosystem properties (transformer species) may enhance
invasion rates in Central European forests. Most tree species
with strong impacts on native vegetation in Central Europe
are strongly controlled by temperature, so climate warming
will increase their potential distribution (Chytrý et al. 2005;
Kleinbauer et al. 2009), and possibly the range of habitats
colonized.
Although the setting of our study sites is context specific (e.g. large adjacent settlements with gardens, lowland
forests), observed low levels of invasion in Central European forests situated at a greater distance to settlements
may turn out to be an ephemeral phenomenon. Spread
of alien forest plants on the landscape level may ultimately, although possibly only over long time periods due
to significant lag times, lead to increased levels of invasion.
Resistance of temperate forests to invasion
Acknowledgements
Open habitats (e.g. industrial, man-made habitats, grasslands) are more receptive to invasions than closed habitats
(i.e. forests; Chytrý et al. 2008). In Europe, temperate forests
(except for forests shaped by natural disturbances, i.e. floodplain forests) are considered to be rather resistant to plant
We are grateful to Christian Schröck and Oliver Stöhr
for their assistance in determining critical specimens. We
are grateful for constructive comments and suggestions of
three anonymous reviewers and the managing editor K. Hövemeyer.
Author's personal copy
8
F. Essl et al. / Basic and Applied Ecology 12 (2011) 1–9
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.baae.2010.
10.003.
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